Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with MTJ technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Similarly, spin-transfer (spin torque or STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has recently stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale. Recently, J-G. Zhu et al. described another spintronic device called a spin transfer oscillator in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry.
Both MRAM and STT-MRAM have a MTJ element based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer, or based on a GMR effect where a reference layer and free layer are separated by a metal spacer. The MTJ element is typically formed between a bottom electrode such as a first conductive line and a top electrode which is a second conductive line at locations where the top electrode crosses over the bottom electrode. A MTJ stack of layers may have a bottom spin valve configuration in which a seed layer, an anti-ferromagnetic (AFM) pinning layer, a ferromagnetic “pinned” layer, a thin tunnel barrier layer, a ferromagnetic “free” layer, and a capping layer are sequentially formed on a bottom electrode. The AFM layer holds the magnetic moment of the pinned layer in a fixed direction. The pinned or reference layer has a magnetic moment that is fixed in the “y” direction, for example, by exchange coupling with the adjacent AFM layer that is also magnetized in the “y” direction. The free layer has a magnetic moment that is either parallel or anti-parallel to the magnetic moment in the pinned layer. The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons. The magnetic moment of the free layer may change in response to external magnetic fields and it is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction. When a sense current is passed from the top electrode to the bottom electrode in a direction perpendicular to the MTJ layers (CPP mode), a lower resistance is detected when the magnetization directions of the free and pinned layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state.
As the size of MRAM cells decreases, the use of external magnetic fields generated by current carrying lines to switch the magnetic moment direction becomes problematic. One of the keys to manufacturability of ultra-high density MRAMs is to provide a robust magnetic switching margin by eliminating the half-select disturb issue. For this reason, a new type of device called a spin transfer (spin torque) device was developed. Compared with conventional MRAM, spin-transfer torque or STT-MRAM has an advantage in avoiding the half select problem and writing disturbance between adjacent cells. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a CPP configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high, and if the dimensions of the multilayer are small. The difference between a STT-MRAM and a conventional MRAM is only in the write operation mechanism. The read mechanism is the same.
Materials with PMA are of particular importance for magnetic and magnetic-optic recording applications. Spintronic devices with perpendicular magnetic anisotropy have an advantage over MRAM devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is a key challenge for future MRAM applications and other spintronic devices.
Materials exhibiting PMA such as CoPt, CoPt—SiO2, Tb(Fe)Co, and FePt have been reported multiple times in publications. However, all of the literature examples suffer from at least one drawback. It is preferred that establishing a PMA property in a spin valve structure does not require strenuous heating. Unfortunately, FePt or Tb(Fe)Co need high temperature annealing to achieve high enough PMA which is unacceptable for device integration since certain components are damaged by high temperatures. CoPt and its alloys such as CoCrPt and CoPt—SiO2 are not desirable because Pt and Cr are severe spin depolarizing materials and will seriously quench the amplitude of spintronic devices if incorporated in the spinvalve structures. That leaves the novel magnetic multilayer systems such as Co/X where X=Pt, Pd, Au, Ni, Ir, and the like for consideration. As stated above, Co/Pt, Co/Pd, and Co/Ir will not be good PMA materials for spintronic devices because of the severe spin depolarizing property of Pt, Pd, and Ir. Furthermore, Co/Pt, Co/Pd, and Co/Ir configurations typically require a very thick and expensive Pt, Pd, or Ir as a seed layer. Au is associated with high cost and easy interdiffusion to adjacent layers which makes a Co/Au multilayer for PMA purposes less practical. On the other hand, a Co/Ni multilayer configuration as a PMA material candidate has several advantages including (a) much higher spin polarization from Co, Ni, and Co/Ni interfaces, (b) better stability from the robustness of Ni layer insertion, (c) much higher saturation magnetization of 1 Tesla or about 2× higher than other Co/M combinations (M=metal), and (d) low cost.
PMA materials have been considered for MAMR applications as described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magn., Vol. 44, No. 1, pp. 125-131 (2008). A mechanism is proposed for recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. FIG. 1 is taken from the aforementioned reference and shows an ac field assisted perpendicular head design. The upper caption 19 represents a perpendicular spin torque driven oscillator for generating a localized ac field in a microwave frequency regime and includes a bottom electrode 11a, top electrode 11b, perpendicular magnetized reference layer 12 (spin injection layer or SIL), metallic spacer 13, and oscillating stack 14. Oscillator stack 14 is made of a field generation layer (FGL) 14a and a layer with perpendicular anisotropy 14b having an easy axis 14c. The ac field generator in the upper caption 19 is rotated 90 degrees with respect to the lower part of the drawing where the device is positioned between a write pole 17 and a trailing shield 18. The writer moves across the surface of a magnetic media 16 that has a soft underlayer 15. The reference layer 12 provides for spin polarization of injected current (I). Layers 14a, 14b are ferromagnetically exchanged coupled. Improved materials for the reference layer and oscillator stack are needed as this technology matures.
Several attempts disclosed in the literature have been made in order to achieve high PMA from Co/Ni multilayer configurations. However, all of the examples typically involve a very thick underlayer to establish PMA. For instance, G. Daalderop et al. in “Prediction and Confirmation of Perpendicular Magnetic Anisotropy in Co/Ni Multilayers”, Phys. Rev. Lett. 68, 682 (1992) and F. den Broeder et al. in “Co/Ni multilayers with perpendicular magnetic anisotropy: Kerr effect and thermomagnetic writing”, Appl. Phys. Lett. 61, 1648 (1992), use a 2000 Angstrom thick Au seed layer. In V. Naik et al., “Effect of (111) texture on the perpendicular magnetic anisotropy of Co/Ni multilayers”, J. Appl. Phys. 84, 3273 (1998), and in Y. Zhang et al., “Magnetic and magneto-optic properties of sputtered Co/Ni multilayers”, J. Appl. Phys. 75, 6495 (1994), a 500 Angstrom Au/500 Angstrom Ag composite seed layer is employed. Jaeyong Lee et al. in “Perpendicular magnetic anisotropy of the epitaxial fcc Co/60-Angstrom-Ni/Cu(001) system”, Phys. Rev. B 57, R5728 (1997) describe a 1000 Angstrom thick Cu seed layer. A 500 Angstrom Ti or 500 Angstrom Cu seed layer with heating to 150° C. is used by P. Bloemen et al. in “Magnetic anisotropies in Co/Ni (111) multilayers”, J. Appl. Phys. 72, 4840 (1992). W. Chen et al. in “Spin-torque driven ferromagnetic resonance of Co/Ni synthetic layers in spin valves”, Appl. Phys. Lett. 92, 012507 (2008) describe a 1000 Angstrom Cu/200 Angstrom Pt/100 Angstrom Cu composite seed layer. The aforementioned seed layers are not practical with Co/Ni multilayer PMA configurations in spintronic devices. Typically, there is a space restriction in a direction perpendicular to the planes of the spin valve layers in advanced devices in order to optimize performance. Seed layers thicker than about 100 Angstroms will require thinning a different layer in the spin valve to maintain a certain minimum thickness for the spin valve which can easily lead to performance degradation.
In particular, it is highly desirable to design a system whereby a thin seed layer helps to establish PMA in overlying layers, and materials exhibiting high PMA may be used to induce partial PMA in magnetic layers with high moments to enable easier FGL oscillation within the high moment materials and therefore generate a larger oscillating field (Hac) for better MAMR performance.